1. Field of the Invention
The present invention is broadly concerned with improved twin screw extrusion devices of a highly versatile nature which can be used for the production of a wide variety of end products of varying densities, cook values and expansion ratios, without the need for extensive machine modifications. The extruders of the invention include a twin screw assembly having non-parallel, tapered conical screws with the flighting of the screws intercalated along the length of the extruder barrel to define close-clearance, preferably constant dimension, alternating upper and lower nip areas and trailing kneading zones and reverse flow passageways; the nip areas create high pressure zones within the barrel which propel material forwardly, while the material is kneaded and allowed to reverse flow in the zones and passageways. In other embodiments, an infinitely variable die assembly including a shiftable stem movable between a waste disposal position and a variety of extrusion positions. A specialized fluid extraction final extruder head is also provided, which allows oils or other fluids to be efficiently extracted, particularly with the aid of a supercritical extractant such as carbon dioxide.
2. Description of the Prior Art
Extrusion cooking devices have long been used in the manufacture of a wide variety of edible and other products such as human and animal feeds. Generally speaking, these types of extruders include an elongated barrel together with one or more internal, helically flighted, axially rotatable extrusion screws therein. The outlet of the extruder barrel is equipped with an apertured extrusion die. In use, a material to be processed is passed into and through the extruder barrel and is subjected to increasing levels of temperature, pressure and shear. As the material emerges from the extruder die, it is fully cooked and shaped and may typically be subdivided using a rotating knife assembly. Conventional extruders of this type are shown in U.S. Pat. Nos. 4,763,569, 4,118,164 and 3,117,006.
Most conventional modem-day extrusion cookers are made up of a series of interconnected tubular barrel heads or sections with the internal flighted screw(s) also being sectionalized and mounted on powered, rotatable shaft(s). In order to achieve the desired level of cook, it has been thought necessary to provide relatively long barrels and associated screws. Thus, many high-output pet food machines may have five to eight barrel sections and have a length of from about 10 to 20 times the screw diameter. As can be appreciated, such long extruders are expensive and moreover present problems associated with properly supporting the extrusion screw(s) within the barrel. However, prior attempts at using relatively short extruders have not met with success, and have been plagued with problems of insufficient cook and/or relatively low yields.
U.S. Pat. Nos. 5,939,124 and 5,694,833 describe short length, high speed cooking extruders which address the problem of excessively long barrel and screw lengths, and thus represent a distinct advance in the art. These extruders, sold by Wenger Manufacturing, Inc. as U P/C extruders, have achieved considerable commercial success.
However, most prior extruders must be designed with screw and barrel section configurations which are specific to a desired product. That is, the configuration used for the production of high density aquatic feeds is generally significantly different than that which would be necessary to produce medium density pet foods or low density feeds. As a consequence, the extruder must be broken down and reconfigured if it is desired to change the product to be produced. Moreover, in some cases an extruder designed for one type of product simply cannot be reconfigured successfully to efficiently produce a significantly different type of product.
Oils such as soybean oil are conventionally extracted from soybeans by mechanical extraction techniques, solvent extraction and/or supercritical fluid technologies. For large production operations, mechanical extractors are inefficient, and the extracted oil requires considerable refinement. On the other hand, supercritical fluid (e.g., CO2) extraction devices are too expensive and complex for existing oil plants. Solvent extraction using hexane or other solvents presents environmental problems associated with disposal of the solvent.
There is accordingly a need in the art for improved extruder equipment of great flexibility and versatility and which can be used to yield dissimilar products without extensive reconfiguration or reworking of the internal extruder components; moreover, improved equipment for the extraction of high quality oils and the like while avoiding the problems of solvent extraction would be an important breakthrough.
The present invention overcomes the problems outlined above and provides a twin screw extruder having an elongated barrel with a material inlet and a material outlet usually equipped with a restricted orifice die, together with specially configured extrusion screws within the barrel. Each screw includes an elongated central shaft having a shaft rear end and a shaft front end with outwardly extending helical flighting provided along the length of the central shaft to provide a flighting rear end, a flighting front end and an outer flighting surface spaced from the central shaft. The central shaft may be of constant diameter but preferably is progressively tapered through a first taper angle along the length thereof from rear to front; similarly, the flighting may be of constant depth but is preferably tapered from rear to front through a second taper angle. Optimally but not necessarily the shaft and flighting taper angles are different, with the latter being greater than the former. Also, the width of the outer flighting surface may be constant from rear to front but advantageously the width changes progressively along the length of the flighting from rear to front; again most preferably, the width of the flighting increases from rear to front so that the width of the outer flighting surface adjacent the front end is greater than the width of the outer flighting surface adjacent the flighting rear end.
The twin screws are positioned in juxtaposition with the central axes of the shafts converging towards each other so that these axes define an included angle. Further, the flighting of the shafts is intercalated, preferably along the entire flighting length. In this fashion, the screws cooperatively define a series of close-clearance, alternating upper and lower nip areas along the length of the screw set. Preferably, the flighting clearance at the respective nip areas is substantially constant along the full length of the screw set, although more generally the nip clearances may increase or decrease along the length of the screw set. The design of the screw set to present the close-clearance nip areas creates a series of high pressure zones within the extruder which serve to positively propel the material being extruded forwardly in a xe2x80x9cpulsingxe2x80x9d fashion.
It has been found that the extruder design affords a high degree of operational flexibility, so that the extruder may be used to produce a variety of products simply by changing the rotational speed of the screw assembly and possibly other processing condition changes (e.g., temperature and die configuration). It has been observed that changes in preconditioning perimeters have a more pronounced effect on the end product, than is common with conventional extrusion equipment. Accordingly, the simple expedient of changing steam and/or water input to the preconditioner can in and of itself significantly impact the properties of the final extrudate.
In another aspect of the invention, an extruder design for extraction of fluids such as oil from oil seed materials is provided. Such an extruder preferably although not necessarily includes the features described above, but includes an extruder head section including an outer shell equipped with a fluid outlet, together with an internal, elongated, slotted sleeve which receives a portion of the extruder screw(s). The sleeve is preferably constructed from a series of elongated bar members which are welded or otherwise affixed together to form a tubular sleeve, with passageways between adjacent bars. The passageways are preferably tapered and present a smaller opening at the interior of the sleeve, as compared with the exterior thereof. In use, an oil seed or other material is passed through the extruder so that in the head section the fluid to be recovered is pressed or extruded through the sleeve passageways. Fluid extraction is materially enhanced by injection of a supercritical fluid such as carbon dioxide or propane into the extruder head section.